1. Field of the Invention
The present invention relates to centering a boom crane on a pedestal and more particularly to a hydraulic control circuit which permits the boom crane to universally rotate in a controlled manner on the pedestal so that it may be centered over the load to be lifted.
2. Description of the Prior Art
In a typical environment for a boom crane operation, the load to be lifted is perhaps 150 to 200 feet from the boom crane operator. Consequently, it is difficult for the boom crane operator to accurately position the boom of the crane directly over the center of the load to be hoisted. If the boom is not centered over the load to be hoisted, the load may swing violently out of control during the initial stages of hoisting, particularly when the crane is mounted on a floating drilling platform where cargo is being hoisted from an adjacent ship. This situation can create an unsafe working environment, particularly when workmen are laboring in close proximity to the load to be lifted.
In recent times crane manufacturers have employed a three-position swing control valve in which the intermediate position of the control valve is a neutral position which fluidly connects the high pressure side of the hydraulic motor to the low pressure side to permit the boom to freely rotate. However, the neutral position of the control valve does not permit the crane operator to control the boom. Consequently, the boom frequently overshoots the center position of the load, particularly when the crane is mounted in a tilted position causing the boom to swing downwardly to its lowest position. In such a circumstance, the hydraulic swing motor would, of necessity, be reemployed in an attempt to center the boom over the load to be lifted. Hence it is difficult for the operator to center the boom over the load to be lifted by engaging the neutral position of the control valve.
Accordingly, there is a need for a hydraulic control system which is capable of restricting the swing speed during the centering operation of the boom over the load to be lifted. Additionally, there is a need for allowing the crane operator to accurately control the boom during the centering operation.